Periodic maintenance
Periodic maintenance is a provision set aside to cover the cost of interim drydocking and special surveys. To maintain class for insurance purposes, all merchant
ships must undergo regular surveys. The ship must be dry-docked every two years
and every four years must have a special survey, approving its seaworthiness. At
its special survey the vessel is dry-docked, all machinery is inspected and the
thickness of the steel in certain areas of the hull is measured and compared with
acceptable standards. The extent of these measurements increases with age. All
defects must be remedied before a certificate of seaworthiness is issued. In older
ships these surveys often necessitate considerable expense, for example in replacing
steelwork that, owing to corrosion, no longer meets the required standards. In
addition dry-docking allows hull maintenance to remove marine growth, which
would otherwise reduce the operating efficiency of the ship.